This invention relates to an apparatus for activating a remote underwater device, and more particularly, for activating said device only upon receipt of a properly coded acoustic wave even though it is in the presence of reverberation, acoustic noise, and multipath.
The activation and control of remote underwater devices by transmitting acoustic waves from a transmitter through the water is limited by the constraints of this acoustic medium and its boundaries. Typically, an acoustic transmitter acts as a point source such that an acoustic wave spreads out ideally in a spherical pattern. Portions of the wave incident upon the air-water boundary and the water-seabed boundary are reflected back into the water medium. These reflections appear at the remote underwater device later in time than that portion of the spherical wave connected by a direct path from the underwater device to the acoustic source. Additionally, portions of the spherical wave are refracted as a function of the salinity, temperature, and depth of the water. The refracted waves also travel a longer distance than the direct path and may appear at the device before or after the direct path wave.
One prior art technique for discriminating between the direct path acoustic wave from the various other portions of the spherical wave was by lengthening the period between successive transmissions until the reflectons, refractions, and reverberations had died out. This naturally lowered the data rate to unacceptable limits.
The actual undersea environment is a mix of natural and manmade noise as well as the multipath and reverberation caused by transmitting acoustic waves therein. Consequently, successive coded acoustic signals may be embedded in acoustic noise such as that caused by ocean wave motion or rain. The signals may be subject to fading due to the refraction and propagation changes of undersea turbulence. Additionally, substitution of a present signal may be made by a prior signal time delayed by the medium. This signal substitution is sometimes termed "leak through".
Reference is made to U.S. Pat. No. 3,422,397 issued to J. A. Lagoe et al. on Jan. 14, 1969. This reference describes a remote underwater device actuated by an incident acoustic wave containing predetermined frequencies including an activating frequency. Such a device is responsive to frequency diversity transmission and space diversity reception to detect the control signals and to reduce the effects of fading. This reference further teaches that a Hamming type digital error detecting code can be used to at least partially protect transmissions from error.
Hamming codes when used in this environment require substantial and complex terminal equipment at both the transmitter and receiver. In this regard, reference may be made to "Logical Design of Digital Computers" by Montgomery Phister, Jr., published by John Wiley & Sons, Feb. 1959, at pages 329 through 336. Additionally, the system of the Lagoe et al. patent appears susceptible to "leak through" and otherwise insensitive to a substitution of one frequency by another with the consequent error effects at the receiver. Lastly, the system of this reference may be falsely actuated by broad-band noise or by intentional frequency sweeping at least on the character if not the word basis.
It is, accordingly, an object of this invention to devise a remote underwater device activating system responsive only to error free received signals and having a substantial capability of discriminating each received acoustic wave from noise, multipath, and reverberation. Relatedly, the activating system should discriminate also between successive groups of received signals and predetermined patterns.
It is, accordingly, another object of this invention to utilize the exclusive frequency diversity form of underwater acoustic signal reception to uniquely distinguish a coded acoustic wave from broad-band noise.
It is a related object of this invention to devise a remote underwater device activating system, which activating system may detect acoustic waves at some distance from the device communicating therewith by some fixed wire means.
Lastly, it is yet another object of this invention to devise an activating system responsive only to successive acoustic waves coded so as to minimize the effects of frequency changes in the encoded signals due to some form of intersymbol influence such as crossmodulation.